MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) induced dose and time dependent cell death in hepatocytes in culture. MPTP was converted to MPP+ (1-methyl-4-phenylpyridinium ion) before cell death occurred. MPP+ when added to the medium also caused cell death. High doses (1 mM) of both compounds caused cell death within 4-6 h while low doses (100-200 MuM) required 24-48 h to induce death. MPTP was taken by the cells and converted to MPP+ by a specific monoamine oxidase (MAO-B). The conversion to MPP+ was markedly reduced by treatment of cells with 10 MuM deprenyl which is a specific MAO-B inhibitor. With the marked reduction in MPP+ production, the early cell death induced by MPTP was completely prevented. MPTP and MPP+ induced a number of cytochemical changes, such as shape change, cell aggregation and cell blebbing, which was observed even before massive LDH leakage. MPTP induced glutathione leakage but only after LDH leakage had occurred. The reason for the unusual delay in glutathione leakage is obscure. MPP+ when incubated with NADPH and NADPH cytochrome P-450 reductase under anaerobic conditions did not produce any radical. However, when incubated in air, MPP+ produced both superoxide and hydroxyl radicals. Whether these reactive oxygen radicals are the cause of cell death remains to be elucidated.